Cube: Bastion. She only takes 1 second to reload in comparison to Scarlet, so you benefit more from the extra bullets.
OL Gear: You want 1 or 2 lines of max ammo capacity to offset her small magazine (20) by default. After that, focus on Elemental DMG and ATK. You can get crit lines as fillers. No need for hit rate since she already buffs herself on burst.